Fibers made from a thermoplastic polymer such as polyester and polyamide are excellent in mechanical properties and dimension stability. Therefore, such fibers are used to manufacture interior accessories, vehicle interior accessories or other industrial products as well as clothing. However, recently, fibers need to have various characteristics according to such various usages. Therefore, techniques are being suggested to provide sensitive effects such as texture and bulkiness to a fiber with a cross section formation. In those techniques, from a viewpoint of controlling a fiber section formation, “making ultrathin fibers” is a mainstream technique having a substantial effect to characteristics of fibers and fabrics made from the fibers.
To make ultrathin fibers, a sole spinning method may achieve only several μm of fiber diameter even if spinning conditions are highly controlled. Therefore, ultrathin fibers are generally made by removing the sea component from sea-island composite fibers made by a composite spinning method. In that technique, a plurality of slightly soluble island components are disposed with soluble sea component in a fiber cross section. The sea component is removed to make the ultrathin fiber comprising the island components after preparing a composite fiber or fiber product. This sea-island spinning technique is often used to manufacture industrial ultrathin fibers such as microfibers in particular. Such a technique is being advanced recently to prepare nanofibers having extreme thinness.
Nanofibers comprising monofilaments which have diameters of several hundreds nm may have a greater material flexibility as well as a greater specific surface area defined as surface area per weight. Therefore, it develops specific characteristics that cannot be achieved by general fibers or microfibers. For example, it is possible that a wiping performance is improved by reducing fiber diameters to increase contact areas and collect dust. In addition, the super specific surface area can improve gas absorption performance, a unique flexible touch (slimy touch) and water absorption performance with microscopic clearances. With such characteristics, nanofibers are used for artificial leathers or textiles having new textures in the apparel field while tight fiber gaps are advantageous to sportswear requiring windbreak and waterproof performance.
However, fabrics made only from nanofibers developing such unique characteristics may be too flexible. Such fabrics may not have a tension or a drape enough to maintain their form. From a viewpoint of mechanical properties, such fabrics can hardly be practically used. Further, nanofibers made from the sea-island composite fibers may have a disadvantage that a processability is greatly reduced in a post process such as weaving process, knitting process and sea-removal process with solvent.
As to those problems, JP 2007-262610 A suggests a mixed yarn consisting of two fibers which have different boiling water shrinkage rates. It is suggested that the mixed yarn should be made by mixing sea-island composite fibers used to prepare ultrathin fibers having an average fiber diameter of 50-1,500 nm together with general fibers having a monofilament fineness of 1.0-8.0 dtex (around 2,700-9,600 nm).
The technique disclosed in JP '610 may improve mechanical properties such as tension and drape, of fabrics relative to another fabric made only from nanofibers by introducing other fibers having greater diameters to contribute the mechanical properties.
However, JP '610 only discloses a technique that the mixed yarn consisting of fibers having greater diameters and sea-island composite fibers is woven and knitted and then subjected to the sea-removal process. With such a technique, the fabric might have greatly biased number density of nanofibers in the cross section direction and the surface direction. As a result, the fabrics disclosed in JP '610 may have a problem that a mechanical property such as tension and drape or a hygroscopicity partially fluctuates. When such a fabric is used to produce clothing such as apparel which directly contacts human skin, the fabric might excessively rub the human skin to unnecessarily damage the skin. Further, the fabric might be wet from sweating to exhibit an unpleasant slimy touch. Thus, the fabric used as a lining cloth to contact human skin may somehow cause an unpleasant feeling.
To prevent a mixed yarn having different fiber diameters from being biased as described above, the sea-island composite fiber can be configured to have island components of different diameters disposed in the sea-island cross section. Such a technique is disclosed in JP '711.
JP '711 suggests a technique about a composite spinneret as an application of sea-island spinneret to form a sea-island composite fiber containing island components having different diameters or section shapes. In that technique, an island component coated with a sea component and another island component uncoated are supplied as a composite polymer flow to a confluence (compression) part in the spinneret. As a result, the island component uncoated with a sea component is fused with adjacent island component to form another island component. This phenomenon randomly occurs to prepare a mixed yarn consisting of thick denier fiber yarns and thin denier fiber yarns. To achieve such a random preparation, the layout of island components and sea component is not controlled in JP '711. Namely, the inserting pressure is controlled to be uniform by a width of flow path provided between separated flow path and introduction hole so that the polymer is discharged from a nozzle at a controlled rate. However, control of the discharging rate may not be sufficient. In other words, to form a nano-sized island component by the technique disclosed in JP '711, the polymer has to be introduced through each introduction hole at the sea component side at a flow rate of only 10-2 g/min/hole to 10-3 g/min/hole. Such a polymer flow rate as an essence of that technique is extremely small and the pressure loss proportional to the polymer flow rate and a wall gap is almost zero. Therefore, controlling the discharging rate may not be sufficient to prevent the nanofiber from having a biased layout. Further, ununiform cross section tends to deteriorate spinnability and might make a partially minimized island component fall off to deteriorate post-formability.
Accordingly, it could be helpful to develop a sea-island composite fiber suitable to prepare a fabric which is excellent in tension and drape with good quality stability and post-formability while hygroscopicity and water absorption performance which are unique to nanofibers are maintained at the same time of preventing a specific slimy touch leading to discomfort.
More particularly, it could be helpful to provide a sea-island composite fiber suitable to prepare a non-conventional high-function fabric excellent in quality stability and post-formability, wherein the sea-island composite fiber consists of two or more kinds of polymers to have a layout of a sea component surrounding island components in a fiber cross section perpendicular to a fiber axis.